The present invention relates to a ribbon speaker wherein a diaphragm thereof is improved to provide an increased transduction efficiency.
A loudspeaker provided in an audio system is an electroacoustic device that converts electric signals (electrical energy) into acoustic signals (sound energy) Electrodynamic loudspeakers are widely used today.
A typical electrodynamic loudspeaker uses a coil and diaphragm arrangement in which a voice coil is fixed at the center of a conical paper diaphragm that is free to move in an annular gap. A strong magnetic field, produced by either a permanent magnet or an electromagnet, is applied across the gap. The audio signal is input to the coil as alternating current, causing it to move in the magnetic field as a result of electromagnetic induction. The diaphragm is thus caused to vibrate at the same frequency as the alternating current and sound waves are produced by it.
Another type of the electromagnetic loudspeaker is a ribbon speaker having a vibrating diaphragm which serves both as the coil and the diaphragm. Referring to FIG. 6, the ribbon speaker comprises a pair of iron yokes 1 and 1a and a magnet 2 provided between the yokes 1 and 1a. A vibrating diaphragm 4 made of a good conductive material is disposed in a magnetic gap 3 between the yokes 1 and 1a so as to oppose the magnet 2. The diaphragm 4 is tensed in the longitudinal direction thereof by a pair of electrodes (not shown). When audio signal (alternating current) is applied to the diaphragm 4 in the magnetic field, the diaphragm 4 is forced to move in a direction perpendicular to those of the magnetic field and the current, as shown by the thumb in FIG. 7 describing Fleming's left-hand rule. As the diaphragm 4 vibrates, sound waves radiates.
Since the diaphragm 4 of the ribbon speaker is directly subjected to the electromagnetic force, causing the diaphragm 4 to vibrate, an extremely improved transduction efficiency is achieved. In the conventional speaker with the voice coil, the voice coil is apt to be partially vibrated, thereby causing irregularity in damping effect dependent on the electromagnetic force. Since the entire diaphragm 4 is vibrated, the damping efficiency is improved in the ribbon speakers. Hence the ribbon speakers is superior in frequency response, transient characteristics and in restraining harmonic distortion, thereby remarkably improving the sound quality.
However, in order to extend the range of the low-frequency response, the width of the diaphragm 4 is increased, so that the magnetic gap 3 is widened. Accordingly, the magnetic flux density in the gap decreases, rendering the efficiency of the speaker to decrease.
The efficiency of the ribbon speaker is inversely proportional to .sub..rho.R .degree.K.sub.R where .rho..sub.R and K.sub.R are specific gravity and electrical resistivity of the material of the diaphragm, respectively. In order to increase the efficiency, it is preferable to use a material having a low specific gravity and low resistivity. That is, if the specific gravity of the material is decreased, a large vibration is obtained at a small magnetic force, since the mass of the diaphragm is decreased. Furthermore with a material having a low electrical resistivity, the resistance of the diaphragm 4 can be reduced without increasing the cross-sectional area and hence the mass thereof, thereby reducing the loss of the audio current due to Joule heat.
Conventional materials that satisfy these requirements are metals such as aluminum, the .rho..sub.R .sqroot.K.sub.R value of which is 4.4.times.10.sup.-1 kg/m.sup.3 .sqroot..OMEGA..m and beryllium, the .sub..rho.R .sqroot.K.sub.R value of which is 3.8.times.10.sup.-1 kg/m.sup.3 .sqroot..OMEGA..m. The other material is, for example, a plastic film coated with aluminum. The aluminum and beryllium have far lower .sub..rho.R .sqroot..sub.R value than other metals such as copper (.sub..rho.R .sqroot.K.sub.R =11.7 kg//m.sup.3 .sqroot..OMEGA..m) and silver (.sub..rho.R .sqroot.K.sub.R =30.1 kg//m.sup.3 .sqroot..OMEGA..m) In particular, beryllium has a lower value than any other existing metal.
However, the efficiency of the ribbon speaker provided with an aluminum or beryllium diaphragm has inferior efficiency, only up to 1 to several percent in those for higher audio frequencies. It is further difficult to realize speaker for reproducing middle audio frequencies, let alone a speaker for lower audio frequencies.
On the other hand, a plastic film coated with aluminum by evaporation or sputtering is advantages in that the mass of the diaphragm 4 is small. However, the non-conductive plastic film increases the resistance of the diaphragm so that, the .sub..rho.R .sqroot.K.sub.R value increases. Thus in principle, the efficiency of such a speaker is lower than the speaker employing the metal diaphragm. The metal coated plastic film diaphragm, with which only inferior efficiency and performance are provided, is virtually unused in practice.
An object of the present invention is to provide a ribbon speaker where the transduction efficiency and the performance thereof may be improved.
According to the present invention, there is provided a ribbon speaker having a vibrating diaphragm disposed in a magnetic gap, comprising the vibrating diaphragm comprising first and second organic films superimposed on each other, and first and second metal films overlaid on the first and second organic films, respectively, the first organic film having a smaller polarization and the second organic film having a large polarization, the first and second metal films being different in work function, a third metal film electrically connecting the first and second metal films, and means for supplying current to the vibrating diaphragm.
In an aspect of the invention, the vibrating diaphragm has a ribbon shape, and the means are a pair of electrodes secured to both ends of the diaphragm.
In another aspect, the vibrating diaphragm has a closed loop, and the means is an exciting coil provided around a part of diaphragm.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings .